1. Field of the Invention
This invention relates generally to channel selecting apparatus for a television receiver, and more particularly, is directed to a channel selecting apparatus for a television receiver having a so-called electronic tuner.
2. Description of the Prior Art
Most existing television receivers employ electromechanical tuners in which a channel-selecting knob is manually rotatable to various positions established by a detent mechanism and which respectively correspond to the twelve VHF channels. These tuners are susceptible to failure because of, for example, faulty detent mechanisms and poor switch contacts. Furthermore, electro-mechanical tuners of the described type become extremely complex and even more unreliable when designed to tune the UHF channels in addition to the VHF channels.
In order to overcome the above problems of electromechanical tuners, so-called electronic tuners have been proposed for television receivers. In these tuners, a varactor, that is, an analog voltage-controlled, variable reactance device, such as, a variable capacitance diode, is employed as the tuning element, and the control voltage therefor is usually obtained either by means of a potentiometer array or a phase-locked loop arrangement. However, the use of a potentiometer array or a phase-locked loop arrangement for controlling the varactor is also unsatisfactory in respect to the cost and/or reliability of the resulting tuner.
The present inventors have developed a channel selecting apparatus for a television receiver having a tuner with a voltage-controlled variable reactance device as its tuning element and which avoids the above described problems, for example, as disclosed in U.S. Pat. application Ser. No. 716,655, filed Aug. 23, 1976, now U.S. Pat. No. 4,085,371, issued Apr. 18, 1978, and having a common assignee herewith. In such channel selecting apparatus, digital or binary codes representing or identifying respective channels are stored or written at respective addresses in a memory, and a digital-to-analog converter provides an analog control voltage for the variable reactance device in correspondence to each digital code selectively read out of the memory in a channel selecting mode of the apparatus. Further, in a programming mode of the apparatus, the changing digitally coded counts of a counter which is counting sweep pulses are applied to the digital-to-analog converter for similarly controlling the variable reactance device and selected counts of the counter, for example, those which result in the appearance on the receiver screen of pictures or test patterns broadcast by selected television stations or channels, are written at selected addresses in the memory as the channel identifying codes. Furthermore, in such apparatus, deviation of the receiving frequency, as established in response to the analog control voltage, from the correct or transmitted frequency for the channel identified by the digital code then being read out of the memory is detected, and, in response to a detected deivation, the digital code being received by the digital-to-analog converter is modified in the sense for removing said deviation, whereby to effect an automatic fine tuning operation. There is also provided a memory rewriting or refreshing circuit operative, when the detected deviation of the established receiving frequency from the correct or transmitted frequency exceeds a predetermined amount, to store the resulting modified digital code in the memory in place of the read out digital code resulting in said deviation so that the receiving frequency established at any time in response to a channel identifying code read out of the memory will remain within the pull-in range of the automatic fine tuning operation.
However, it is possible in the last-described channel selecting apparatus to effect writing or rewriting of digital codes in the memory even when such digital codes do not correspond to the transmitting frequency of a broadcast channel receivable by the television receiver. Thus, for example, small children when playing with the controls of the television receiver may write or store, at the several addresses of the memory, digital codes that bear no relationship to the frequencies with which television signals are broadcast for the channels receivable at a particular locale. After such erroneous programming or reprogramming of the memory, the reading out of the stored digital code from a selected one of the memory addresses in the channel selecting mode of the apparatus will not tune the receiver to the receiving frequency for a selected channel.